I enjoyed the book and I think it will actually be helpful to me even though I'm not practicing zen. Nice work! I do wish there'd be more on breathing practices though.

BTW, do all the breathing practices in Rinzai lineage originate from Hakuin or are/were there other such practices before him too?

Also is there any place where I could read more on the ba field? I didn't know this concept existed in Buddhism but I heard of something similar in one Daoist tradition and also probably experienced it myself with some of my teachers (especially my root teacher) and some older students.

Although many individuals in this age appear to be merely indulging their worldly desires, one does not have the capacity to judge them, so it is best to train in pure vision.
- Shabkar

I will probably write something in the future containing more extensive breathing/internal cultivation information. My motivation for doing so would not be instructional, but just to reveal some of the wealth of the tradition that, for a few reasons, remain largely unknown to practitioners in the West.

Hakuin certainly promulgated certain practices, i.e. his famous nanso and naikan methods. His student Torei, who incidentally also had a background in Shinto practices that may have informed his presentation, did so as well. But there are records recording this emphasis on cultivation centered on the kikai tanden and so on going back to the earliest transmission of Chinese Linji Chan lineages to Japan, in the the Kamakura period. Beyond that I'm out of my depth.

Re ba, I don't really have anything to recommend for reading. But it's really just a way of describing how one's conditions are affected in the proximity of a realized person. If described in more detail, we often talk about joriki (samadhi power), kiai (energetic quality) and how these things affect others. One of the teachers connected to our lineage described something like this in a different context, but it fits here as well: he said "When kiai (energy) strikes kan (the intuition, knowing, or clear seeing arising from realization) a wondrous (myo) sound emerges", that is, the spontaneous, wondrous functioning of infinite skillful means can manifest.

Some more Zen talk there for you, as if the book wasn't already too much

~ Meido

It is relatively easy to accomplish the important matter of insight into one’s true nature, but uncommonly difficult to function freely and clearly [according to this understanding], in motion and in rest, in good and in adverse circumstances. Please make strenuous and vigorous efforts towards this end, otherwise all the teachings of Buddhas and patriarchs become mere empty words. - Torei